Frequency measuring system



July 27, 1965 R. R. ARIESZ FREQUENCY MEASURING SYSTEM 2 Sheets-Sheet 1 Filed Deo. 29, 1944 nl EV v @www TTOR/VEV /NVENTOR R. R. @f-52 BV July 27, 1965 R. R. R11-:sz

FREQUENCY MEASURING SYSTEM 2 Sheets-Sheet 2 Filed Dec. 29, 1944 .222.22m $22.222 .2.32 23 2.32 222m wwwh 23E ..0225 21.2 Z w um@ 2|. SRM 1| .32 1 1. 3&1 @m3 1m 2.2222222 eww L $.22 3N \111 R5 l 122m 111 l 11 1 1 1Q l .2222 51ml OE P21111 m22 @2.2 222 T .1H llllllllll 1 1 Qwwnw 1 2 2.222222 1 11 H1 EN.. R 22223 11 V QQBWNQ Q TI l l l l i 1 l l .|11 2 QMQN1QMN` 1. b .Nwb l 1 l l l l l l l 1 |11 111111 .n.222 .2.22 .H 11|I 11 21521322 1 m 2.22212 1111.11.11ll1111111 H111. 1.1111 lllllllllll 1 1 222.213,22 1 v .21.222222 11111111111111 1 1 11111 Vf E S2 1. B .111111 11 lllllllll 11 222132 1 n .G2233 11111111111111 11 111. .1H 1 1 1 1 I 1 1 1 1 1 1 1 @um nu 1 w 2.222225 1 1 1 1 l 1 1 1 1 1 1 1 1 1 1 1 H.. Q .|11 20mm I GQ WN Ruhr P69. 1&.2 1l 111 IESMQ 2 m2222222 1o I. .2222 -mm2 Rui 22 /92 2.2 3.222222 2292.221222.

2Q a QQ/ .222 emana 2F22 222.222 222.22622 w22 .22 ..2 3.319 3x2 2G22 wcmmm. WMV@ Qh EQWW @wsdl www 111121352. 222.22m |11 1&2 1m2h2 d 20.2 222232.22 m 1222 n.. d* @MK2 kum@ 11 MWMWLMRM 222.22m .owl 2m\ .2222 mm .22 um 2 2.2 12% H8 .www2 .$2.2 .121.2

Nk 222123 2.12. 1 e v 22x 2 Y 21.2.2 .222 wwvncm .G12 229.2202222 .32223 IHN H wm l 22.22223 222m L11 122.2 LUQMM .BQ .11 @bkk .NY 1111. Q `v2 m2 l 22.0.2 2.2 2222. 22122222222 Qwo; kh .|1111 Y2 m2 Q22 WIM qhu GQWQ ptl s; QMN Gwkl 1w QMNLY United States Patent O Robert R.

nu. 2a, isis, ser. No. sialic? s l ici. irisi) The invention relates to a frequency response device and particularly to a fr suencv measuring or pulse counting circuit to a system for analyzing and synthesizing speech.

An object o the invention is to provide a precise measurement of the frequency of electrical oscillations, for example, of the fundamental Yrequency of a repeated lectric signal wave, such as speech wave.

A related object is to produce a direct current voltage which is `proportional to the frequency or" an alternating signal wave over c. wide rango of frequencies.

@ther objects are to derive a slowly varying direct current signal from a speech wave, the magnitude of which is proportional to the fundamental pitch of the speaers voice; and to properly control the operation of a irequency measuring circuit or counter in the pitch control channel of a s eech analzinf7 and synthesizing system of the vocoder type.

in accordance with the invention these objects are attained efficiently by a simple and economical circuit arrangement employing a single-trip vacuum tube multivibrator of novel design, which is triggered by sharp voltage pulses produced by transrrdtting a wave of the frequency to be measured thro..gh a limiting amplifier and a condenser-resistance circuit o very small tirne constant, adapted to produce a direct current output the average value of .viiich is linearly proportional to the frequency of the input wave over a Wide frequency range and which is substantially by changes in the amplitude and shape of that Wave.

A feature of tl e invention is a control circuit for the frequency measuring Lrcuit or counter in the pitch control channel oi a vocoder systen., which discriminates between voiced certain unvoicecl sounds, and acts as a control on the time oli deriv on of the pitch signal.

various features ob cts of the circuits of the invention will be better understood from the following detailed description when read in coniunction with the acco .nying draafings in which:

FiG. l shows schematically a frequency measuring or pulse counting circ it embodying the invention;

FG. 2 shows curves used to explain *le operation of the circuit of FlG. l; and

FlG. 3 shows scheinaticaly a speech analyzing and synthesizing system of the type known as a vocoder employing a pitch deriving cli r1 el and a control therefor enibodyin the invention.

In the circ of FG. l, alternating signal wave of the frequency to be measured, such as a speech Wave, which may be of arnental sine Wave torni such as shown by the ide lized curve e of PEG. 2, received from a source S is esse-d thro-gh input transformer T on input of a lirn Lili of vieil-known type. As shown, this r r comprises two three-electr de amplifying vacuum tube stages VT and VTZ, which rnay be contained in a single envelope, coupled in cascade through a resistance-condenser cou g circuit CC, the hrsL stage VTi including a large resistance R in series with its grid circuit, for providing a desired amount or" ainpiiicat by the selected circuit constaats, and 'Fo i r eiiectively removing amplitude variations from the applied signal Wave by cutting off its positive and negative voltage peaks. With a sufficiently high input signal level this result could be produced with a single lbfib@ Patented july 27, i965 fing stage.

LAE is transmitted through a circuit compris- .e c rfclenscr Ci and the resistance R2 in series, the values of which are selected to provide a very small time constant. This, in effect, develops a voltage across the resistance l of that circuit which is he derivative of th applied square-shaped Wave and contains in successive half cycles a very sharp positive and a negative voltage peak or spike due to the charging and discharging currents to Cl Lbrouggh R2. This peaked voltage Wave shown by idealized curve c of PEG. 2, is utilized for triggering oilC a "erip multivibrator MV operating in combination with the meter M and the low pass lter F in its direct current output circuit as a frequency or pulse counter.

The multivibrator MV includes the two three-electrode ampliiying vacuurn tubes VTS VT-, which may be contained in a single envelope as shown. The anode or plaie of tube VTS is coupled to the control grid of the tube V i5:- is connected to the control grid of the tube resistor circuit oi sino;

urne constant. The catbodes of the tubes VTT; and V Yi may be heated to incandescence from any suitable source (not shown), which may be a or .t cu battery. The control gr catho-:le circuit of the Ue Vle comprises the resistor Ril connect d between its cathode and ground, the negative grid biasing battery E; and the resistance i 2 in series. The control grid of the tube Vi is connected to the cathode of that tube thro gli Ythe series resistor R3. Plate Current is supplied from the cornrnon plate battery in parallel to the anode-s oi the tubes VTS and VT@ through the individual equivalent resistances R6 and R7, resnecti ely. The in. r M, which rnay be an animeter, is connected across the cathode resistor of the multivibrator tube Vm large (28,660 ohrns) series resistor R3 1J nccted between the plate of that tube and the grid of the tube VTS, is charged positively with respect to ground by p e battery throuvh resistance R6. Also, with no signal input to the inultivil rato-r MV, the control grid of the Looe VT4 has zero bias so that this tube is in the conducti f condition and its anode voltage is low. For that con on, the posi 've voltage drop developed across resistance R2 is sinall.

When a sharp positive voltage peak is produced across the resistance R2, the positive voltage applied to the grid of the multivibrator tube VT3 will be suliciently large to overcome the negative bias voltage supplied by battery Bl and that tube will suddenly become conducting. This will abruptly reduce the anode voltage of tube VTS, which in turn, will cause the condenser CZ (which was positively charged from battery B2) to discharge through the circuit comprising the resistances R3 and R4 and the discharge path of the tube VT3 in series. The resulting negative voltage produced across the resistance R3 in the grid circuit of the tube VT4 will be impressed on the grid of that tube driving it beyond cut-oil. The resulting increase in the anode voltage of tube VT4 will cause a Y. 3 positive-voltage to :appear across the resistance R2 due to its connection to the anode of that tube through resistance R5. This positive voltage across resistance R2 applied to the control grid of tube VTS will maintain that tube in a conducting condition as long as the tube VT4 is non-conducting, even though the positive voltage pulse vacross the resistance R2 due to the charging of the condenser C1, is no longer present. After the elapse of a certain time interval determined by the time constant (RS4-R4) and C2, the negative voltage across RFB will have been reduced to the point that tube VT4 again becomes conducting. The resultant reduction in its anode voltage will reduce the positive voltage across the resistance R2 and thus the positive potential on the grid of the tube VT3 so that the latter tube abruptly becomes nonconducting, and the resulting increase in its anode voltage will allow condenser C2 to be again positively charged with respect to ground through resistance R6 from plate battery B2. The multivibrator MV is thus returned to its normal unoperated condition and will remain in that condition until another positive voltage peak in response to the input signal wave appears across the resistance R2, when the cycle of operations just described will be repeated.

During the timethat the tube VTS is maintained in the conducting condition which, as stated above, will be as long as the tube VT4 is maintained non-conducting in stant of the resistances R3 and R4 and the condenser C2. Y

The shape of this pulse will not be affected by variation in the amplitude and shapeV of the input signal wave. The shape of the voltage wave produced across the cathode resistor R4 of the multivibrator MV is shown by the idealized curve d -of FIG. 2. The average value of the direct currentcomponent of the voltage across the cathoderesistor R4 will increase linearly with input frequency up to such a maximum frequency that the interval between the positive and negative voltage pulses appearing across the resistance R2 in response to the applied square wave, is equal to the duration of the voltage pulse across the resistance R4. Further increase in input frequency will not resultin an increase in output voltage.

The voltage thus produced across the cathode resistor R4 will cause pulses of current to be transmitted through the low-pass filter F to the meter M, the function of the low-pass filter being to effectively remove instantaneous variations or ripples in that current. To accomplish this, the cut-olf frequency of the filter is properly selected with respect to the frequency of the input wave. The deflection of the meter M will be a function of the average current applied to it, and thus will be directly proportional to the frequency of the speech wave applied to the input of the frequency measuring'circuit.

Among the practical advantages of the frequency measuring circuit of the invention as described above are the following:

(1) Accurate frequency measurement is attained with a simple circuit arrangement and a relatively small amount of transmission apparatus;

(2) The average value of the output voltage is proporcathode resistor in the multivibrator enables the negative Y 4 side of the output to be on ground with no current flowing, when no signal is applied.

FIG. 3 shows a vocoder system of the general type disclosed in -my US. Patent No. 2,183,248, issued December 12, 1939, or in the Dudley Patents No. 2,151,091, issued March 2l, 1939, and No. 2,243,527, issued May 27, 1941, which may be utilized for analysis and synthesis of speech or other vocal sounds, frequency range reduction and restoration or for other purposes as set forth in the patent specifications, employing a pitch deriving channel and control therefor embodying the invention.

In FIG. 3, the analyzer at the transmitting end of the system is shown on the left and the synthesizer at the receiving end of the system on the right.

The analyzer includes a telephone transmitter or microphone TT for transforming applied speech sounds into an electrical wave comprising a continuous band of speech frequencies, say, of the frequency range, 0-3000 cycles per second. The electrical output of the transmitter TI' is amplified and adjusted to a constant volume by the vogad (voice operated gain adjusting device) VD, which may be of any of the Well-known types. Equalization of the speech energy over the frequency band is preferably obtained by means of a predistortion equalizer of suitable design, which may be incorporated in the vogad amplifier. Following the vogad, the circuit branches into two main parts; the rst branch Vconnecting'to the transmitting circuits of a plurality of spectrum channels, shown at the bottom of the figure, which extend through to the synthesizer at the receiving end of the system; and the second branch connecting to the pitch deriving or frequency pattern con-V trol channel, shown at the top of the figure, which extends through the energy source for the synthesizer at the receivingend of the system.

In the spectrum channels 1`to S, the speech wave is divided by the band-pass filters F1 F8 in the inputs of the individual channels into eight contiguous frequency `subbands. The eight frequency subbands in the system illustrated, comprise 'the frequency ranges, 150-550, S50-850, S50-1150, 1150-1450, 1450-1750, 1750-2050, 2050-2350 and 2350-2950 cycles per second, The selected frequency subbands Iare amplified and rectified by the amplifier-rectifiers AR1 ARS `in the respective channels 1 8; and the resultant direct current components in these channels are extracted by the respective low-pass filters F9 F16 each passing the frequency range 0-25 cycles per second. The output of each lowpass filter F9 F16 is a unidirectional current varying in amplitude at a syllabic rate, the values of which at any instant is a function` yof the energy content of the particular frequency subband of vthe channel with which the low-pass filter is associated. The outputs of the lowpass filters F9 F16 in the transmitting circuits of the spectrum channels are transmitted to the receiving or synthesizing end lof the system where they are respectively applied -to the signal input terminals o-f the identical modulators M1 M3 in the receiving spectrum circuits, corresponding to the shaping networks SNI SNZ in the individual receiving spectrum circuits of the system shown in FIG. 1 of my aforementioned patent.

The pitch deriving or frequency pattern control channel of the system of FIG. 3 consists Iof two main parts: (1) the pitch deriving channel proper, shown at the top of the figure; and (2) a control therefor, shown just rbelow it at theanalyzing end `of the system. As shown, the pitch deriving channel proper includes in its input a band-pass filter F17, the function of which is to extract from thespeech Waves supplied from the output of the vogad VD the fundamental frequency of the voice, that is, the fundamental voice pitch frequency. The bandpass filter 17 should only pass a band of frequencies equal to the fundamental speech frequencies to be handled by the vocoder, which is usually considered tfo be about from to 400 cycles per second, las indicated. Following the filter F17 is an amplifier A1 utilized to raise the amplitude of the pitch signal to the necessary value; a variable attenuator or vario-losser VL forming a part of the control for the pitch deriving channel, to be described below; and a frequency measuring circuit, including in order a limiting amplifier LA, a frequency counter FC and a 0-35-cycle low-pass filter F18 corresponding to the similar elements lin the frequency measuring circuit of FIG. l, as described above, the function of which is to convert the fundamental pitch frequency to a slowly varying direct current signal the magnitude of which is proportional to the fundamental pitch of the speakers voice.

The pitch signal so derived is transmitted over a wire path to the receiving end of the system, where it controls the energy source ES of the synthesizer, such as is described in detail in the aforementioned patents, to produce a flat spectrum of either continuous or discrete frequency components representing the frequency .patterns f the original speech signals. Appropriate frequency suboands respectively having the same frequency range as passed by the band-pass filter in a different -one of the spectrum channels l to S are filtered by the energy bandpass filters F19 26, respectively, from the output of the energy source ES, and are respectively supplied as control currents to the carrier input of an appropriate `one of the modulators Mii M8 in the receiving circuits of the spectrum channels l to 8. ln each channel modulator circuit, the control current produces an unbalance which .allows a proper amount of energy from the synthetic speech source to be transmitted to one of the receiving bandpass filters F27 F34, for the frequency band transmitted by each spectrum channel, the latter filters providing addi-tional frequency selection. The selected frequency' subbands in the outputs of the filters F27 F34 are superposed in the inputs of the receiving amplier RA to form a Wave simulating the original speech wave applied to the input of the system. This Wave, after amplification in the receiving amplifier RA to the desired listening level, and restoration to the correct frequency composition by means of incorporated equalization in that amplififier, is transformed into speech sounds by the telephone receiver TR.

The proper operation of the speech analyzing and synthesizing or vocoder system which has just been described requires proper control of the time when the 'pitch signal is to be derived and discrimination between voiced and unvoiced sounds in the signals supplied to the system by the pitch deriving channel. This control is obtained by a circuit including the vario-losser VL located in the pitch deriving channel in front of the frequency counter FC, and an enabling branch circuit EC and a disabling branch circuit DC for proper-ly controlling the loss value of that vario-losser. The vario-losser VL comprises a short section of transmission line coupled by an input transformer T2 and an output transformer between the output of ampliher Al and the input of the limiting amplifier LA, with the copper oxide rectiiiers l, 2 pole-d as indicated, in series with the respective sides of the line section, and the linear resistances 3, l respectively, shunted across the line section on the input and output sides of these rectiiiers. The elements of this vario-losser are selected so that it has about 40 decibels more loss in the normal or non-operated condition than when an enabling voltage of about 2 volts direct current is applied across the mid-.point or the shunt resistances 3 and 4f by operation of the enabling circuit EC in the manner to be described below. With this additional loss, the amount of limiting in the following limiting amplifier LA is made insufiicient to operate the frequency counter FC, even though a pitch signal or" nor-mal amplitude is present in the output of the amplifier Al.

The enabling branch circuit EC for the vario-losser VL includes an amplifier AZ in its input for amplifying the applied portion of the speech Wave output of the vogad VD, followed in order by the enabling band-pass filter FSS adapted to pass a frequency band between about 150 and 900 cycles per second, which is the frequency region in which the voiced sounds in speech are of maximum amplitude; the enabling rectifier ER for rectifying the speech wave components passed by the filter F35; and the enabling low-pass filter F36 passing the frequency range -SO cycles having its output connected .across the midpoints of the shunt resistances 3 and 4 in vario-losser VL. The diabling branch circuit DC includes in its input the high-pass disabling filter F37 for transmitting all frequencies applied to its input from the output of the vogad VD, in the frequency range about 4500 cycles, which is the frequency range in which the unvoiced sounds are usually of maximum amplitude, followed by the one-way amplifier A3 and the disabling .rectifier DR for amplifying and rectifying the waves transmitted by the disabling filter. The output of the disabling rectifier DR and the output of the enabling rectitier ER are applied to the input of the enabling lowpass filter F36 in opposing polarity.

The elements of the enabling circuit EC are selected so that a normal output for the enabling low-pass filter F37 of about 2 volts direct current is obtained for sustained voice sounds applied to the analyzer input, which voltage, as stated above, when applied as a bias to the vario-losser VL, is effective to change the resistance of the rectifier elements ll, 2 therein so as to remove a loss of about 40 decibels from the fundamental pitch signal deriving channel. This provides a signal amplitude at the output of the limiting amplifier LA which is sun"icient to operate the frequency counter FC. In general, the voiced sounds have spectra which contain more energy in the low frequency region than in the high frequency region so that, independently of the absolute level, the output of circuit EC will for voiced sounds overcome the output of the disabling circuit DC and will apply a voltage in a direction to reduce the loss in the vario-losser VL and allow the frequency counter FC in the pitch deriving channel to operate,

On unvoiced sounds, which have nearly fiat frequency spectra, the low frequency energy is the same, or less, .than the high frequency energy, so that for such sounds the output of the high frequency disabling filter F37 in the disabling circuit DC will just balance or will overcome the output of the enabling band-pass filter F35 in the enabling circuit EC. In either case, the normal high loss condition of the varioelosser VL will be maintained to prevent operation of the frequency counter FC in the pitch deriving channel on such Sounds.

Since it has been found that the enabling circuit EC functions best on a signal of hat speech rather than on one of predistorted speech, where Ithe predistortion network is included in the Vcgad, to provide optimum results, it may be desirable to connect the input of the enabling circuit EC to the output of the gain decreaser amplifier in the vogad VD rather than to the output of the voag VD, as indicated in FIG. 3.

Various modifications of the circuits illustrated and described which are within the spirit and scope of the invention will be apparent to persons skilled in the art.

What is claimed is:

i.. in combination with a pitch control channel for a speech analyzing and synthesizing system, including means for transforming speech sounds into a constant volume electrical wave comprising the same frequencies, filterirg means for selecting a narrow frequency band contair'ng the range of fundamental speech frequencies from said Wave and a frequency counter for converting the selected band into a slowly varying direct current signal whose magnitude is proportional to the fundamental pitch of the speakcrs voice, a circuit for controlling the time of duration of the pitch signal comprising a variolosser in the input of said frequency counter, normally providing a loss sutiicient to prevent operation of said counter in response to the applied waves, an enabling branch circuit including a filter for selecting from, said vfrequencies in the low frequency region in which the voiced sounds usually contain a greater amount of energ than do the unvoiced sounds, means for rectifying the latter selected band and for applying the rectified voltage to bias said vario-losser in such direction as to reduce its loss, a disabling branch including another filter for selecting from said constant volume electrical Wave a third band of speech frequencies in the higher frequency region in which the unvoiced speech sounds have substantially constant energy and means for rectifying the selected third band and applying the rectified volt- 'age to bias said vario-losser in such direction as to increase its loss, and means for relatively adjusting the gain of said enabling and disabling circuit branches so rthat for voiced sound inputs to said system theresultant bias applied ,to said vario-losser Will be such as to reduce its loss sufficiently to enable operation o-f frequency counter, and for unvoiced sound inputs only to said system the resultant bias applied to said vario-losser will be such fas to maintain its'loss sufficiently high to prevent opera- Ition of said'frequency counter.

2. The combination of claim '1, in Which-said filter in said enabling branch circuit is a band-pass filter passing frequencies in the range between 150 and 900 cycles per second, and said filter in said disabling branch circuit is a high-pass filter passing frequencies above 4500 cycles per second. l

3. A pitch deriving circuit including means for transforming applied speech sounds of a range of frequencies into an electrical Wave having the same frequencies and of substantially constant average amplitude level,. filtering means for selecting from said Wave a narrow frequency band-containing the fundamental speech frequencies, a frequency counter supplied with the selected band, for

converting that band into a slowly varying direct cur- `rent the magnitude of which is proportional to the fundamental pitch of said speech sounds, a vario-losser in the input torsaid frequency counter for controlling the sensi'- tivity thereof and a control circuit therefor comprising an enabling circuit branch and a disabling circuit branch, said enabling circuit branch kcomprising lmeans for deriving a voltage from the speech frequencies in said electrical Wave in the low frequency region'in which the voiced sounds usually contain a greater arnountof energy than the unvoiced sounds and for applying said voltage as a bias to said vario-losser in such direction as to cause its loss Value to be reduced, said disabling circuit branch comprising means for deriving Va second voltage from the speech frequencies in said electrical Wave in the Vhigher frequency region in which the unvoicedv sounds have substantially constant energy, and for applying said second voltage Vas a bias to said vario-losser in suchdirection as to cause its loss to be increased, the constants Yof said vario-losser, said enabling circuit branch and said disabling circuit branch being selected so that for voiced sound inputs to said frequency counter, the resultant bias on saidV vario-losser causes the loss thereof to be sufliciently 10W to enable operation of said frequency counter by the applied waves, and for unvoiced sound inputs only to said frequency counter, the resultant bias on said vario-losser causes the loss there- Vof to be suticiently high to prevent operation of said frequency counter by the applied waves.

References Cited by the Examiner UNITED STATES PATENTS 2,260,933 ,1o/41 CooperV 324-78 2,325,927 s/43 Wilbur 324-70 2,366,076 12/44 Wiibur 324-78 OTHER REFERENCES Article by Lenihan, Pulse Generation in Electronic Engineering, March 1944, pp. 408-411.

ROBERT H. Rosa, Primary Examiner. 

1. IN COMBINATION WITH A PITCH CONTROL CHANNEL FOR A SPEECH ANALYSING AND SYNTHESIZING SYSTEM, INCLUDING MEANS FOR TRANSFORMING SPEECH SOUNDS INTO A CONSTANT VOLUME ELECTRICAL WAVE COMPRISING THE SAME FREQUENCIES, FILTERING MEANS FOR SELECTING A NARROW FREQUENCY BAND CONTAINING THE RANGE OF FUNDAMENTAL SPEECH FREQUENCIES FROM SAID WAVE AND A FREQUENCY COUNTER FOR CONVERTING THE SELECTED BAND INTO A SLOWLY VARYING DIRECT CURRENT SIGNAL WHOSE MAGNITUDE IS PROPORTINAL TO THE FUNDAMENTAL PITCH OF THE SPEAKER''S VOICE, A CIRCUIT FOR CONTROLLING THE TIME OF DURATION OF THE PITCH SIGNAL COMPRISING A VARIOLOSSER IN THE INPUT OF SAID FREQENCY COUNTER, NORMALLY PROVIDING A LOSS SUFFICIENT TO PREVENT OPERATION OF SAID COUNTER IN RESPONSE TO THE APPLIED WAVES, AN ENABLING BRANCH CIRCUIT INCLUDING A FILTER FOR SELECTING FROM SAID CONSTANT VOLUME ELECTRICAL WAVE A SECOND BAND OF SPEECH FREQUENCIES IN THE LOW FREQUENCY REGION IN WHICH THE VOICE SOUNDS USUALLY CONTAIN A GREATER AMOUNT OF ENERGY THAN DO THE UNVOICED SOUNDS, MEANSD FOR RECTIFYING THE LATTER SELECTED BAND AND FOR APPLYING THE RECTIFIED VOLTAGE TO BIAS SAID VARIO-LOOSER IN SUCH DIRECTION AS TO REDUCE ITS LOSS, A DISABLING BRANCH INCLUDING ANOTHER FILTER FOR SELECTING FROM SAID CONSTANT VOLUME ELECTRICAL WAVE A THIRD BAND OF SPEECH FREQUENCIES IN THE HIGHER FREPQENCY REGION IN WHICH THE UNVOICED SPEECH SOUNDS HAVE SUBSTANTIALLY CONSTANT ENERGY AND MEANS FOR RECTIFYING THE SELECTED THIRD BAND AND APPLYING THE RECTIFIED VOLTAGE TO BIAS SAID VARIO-LOOSER IN SUCH DIRECTION AS TO INCREASE ITS LOSS, AND MEANS FOR RELATIVELY ADJUSTING THE GAIN OF SAID ENABLING AND DISABLING CIRCUIT BRANCHES SO THAT FOR VOICED SOUND INPUTS TO SAID SYSTEM THE RESULTANT BIAS APPLIED TO SAID VARIO-LOOSER WILL BE SUCH AS TO REDUCE ITS LOSS SUFICIENTLY TO ENABLE OPERATION OF FREQUENCY COUNTER, AND FOR UNVOICED SOUND INPUTS ONLY TO SAID SYSTEM THE RESULTANT BIAS APPLIED TO SAID VARIO-LOOSER WILL BE SUCH AS TO MAINTAIN ITS LOSS SUFFICIENTLY HIGH PREVENT OPERTION OF SAID FREQUENCY COUNTER. 